Polymer blends can be formed or shaped into lightweight and durable articles such as automobile parts, toys, and housings for various types of equipment. Polymer blends such as those with polypropylene and ethylene-propylene copolymers, however, are difficult to form into products which are simultaneously lightweight, easily processed into large parts and have surfaces which are durable and adherent to paints.
Many attempts have been made to enhance the properties of products formed of polymer blends such as those formed of propylene-ethylene copolymers. Japanese patent publication Nos. 57-57049, 62-5460 and 62-5461 each attempt to do so by improving the fluidity and stiffness of blends of polypropylene with ethylene-propylene copolymer. In addition, Japanese patent publication No. 61-19651 also employs blends which have a large amount of propylene.
The propylene-ethylene copolymer blends shown in the above publications suffer from relatively slow crystallization rates. As a result, longer cooling periods are required to manufacture injection molded products. These blends, moreover, do not necessarily provide products which have excellent durability, such as surface smoothness and surface hardness.
Japanese patent publication No. 60-3420 shows a polymer blend which includes polypropylene, ethylene-propylene copolymer, talc and propylene-ethylene block copolymer. This composition is said to show adhesion for coatings as well as low temperature impact resistance.
Japanese patent Laid-Open publication No. 1-204946 shows a composition comprising ethylene-propylene copolymer, polyethylene, ethylene copolymer, polypropylene and talc. This composition is said to provide products which have dimensional stability.
European patent applications EP 0 519 752 A2 and EP 0496 625 A2 each show blends which comprise ethylene copolymer, propylene polymer and propylene-ethylene "block" copolymer and talc. These blends are said to have good processability for injection molding. Ease of processability is often associated with low melt viscosities (high melt flow rates). These high melt flow rates are often detrimental to other physical properties such as impact strength at low temperatures. Injection molded products formed of these blends are said to have good appearance, low density, good surface hardness and impact resistance, as well as good adhesion.
The above mentioned blends are said to have a morphological structure comprised of crystalline domains embedded in amorphous matrix, contrary to conventional thermoplastic polyolefin blends in which elastomer domains are dispersed in a polypropylene matrix. This structure, however, still has regions where the concentration of elastomers is significantly higher than in surrounding areas, thus effectively exhibiting the existence of elastomeric "macrodomains". These macrodomains have an average size of greater than 4 .mu.m, which may negatively effect certain physical properties of the blend.
Although the compositions shown in the aforementioned references are said to possess good processability, products formed of these compositions tend to be deficient in one or more physical properties such as durability and paint-adhesion. A need therefore continues for thermoplastic blends which can be processed to provide products which have combined properties of superior surface hardness, impact resistance, processability, flexural modulus, adhesion of coating, and which can be painted using conventional techniques.